Ceramic mirrors and method of manufacturing same

ABSTRACT

A ceramic mirror having a maximum surface flatness of 1 μm and/or a maximum centerline average height of 1 nm as measured by the laser interference method, and a method of manufacturing a ceramic mirror by compacting a titanium oxide powder having an average particle diameter no greater than 1 μm and a purity of at least 99% and then sintering the compact in an ordinary, inert, vacuum, or reducing atmosphere at a temperature between 1000° and 1300° C. The sintering pressure may be a minimum of 50 kg/cm 2 . After sintering, the product may be further treated by hot isostatic pressing (HIP) in an Ar atmosphere at 1000° C. and a pressure of 1800 kg/cm 2 . The sintered bodies are then surface ground or lapped to produce ceramic mirrors having the above characteristics for use in laser and X-ray apparatuses, cameras, and other precision optical instruments.

This is a continuation of application Ser. No. 08/064,044, filed May 20,1993, which was abandoned upon the filing hereof.

BACKGROUND OF THE INVENTION

This invention relates to ceramic mirrors for use in laser and X-rayapparatuses, cameras, and other precision optical instruments and thelike and also to a method of manufacturing the mirrors.

Mirrors for use with precision optical instruments and the like arepredominantly made of glass, ruby, sapphire and the like, and also ofsingle crystal ceramics. In recent years alumina and SiC have partlycome in use as polycrystalline ceramics.

Glass-based mirrors are made relatively thick to make up for theinadequate strength and fragility, and therefore the heavy weight andlarge size of the mirrors as a whole have been a problem.Polycrystalline alumina, SiC, and other ceramic materials are used inthe form of sintered bodies after grinding and lapping. None of themhave proved fully satisfactory in respect of the properties such assurface smoothness and optical properties, because the pores inherent tothe ceramic materials remain exposed on the lapped surfaces.

BRIEF SUMMARY OF THE INVENTION

It is an object of the invention to provide a mirror having excellentsurface smoothness with fewer pores than heretofore known and havingsuperior optical properties due to lower irregular reflection of light.

The present inventor, in an effort to solve these problems, sintered ahigh-purity, fine titanium oxide powder at a relatively low temperature.This has been found to give a mirror having excellent surface smoothnesswith a smaller number of pores than before and having superior opticalproperties due to less irregular reflection of light. The discovery hasled to the perfection of the present invention.

The invention resides, in essence, in a ceramic mirror characterized bya surface flatness of no more than 1 μm and/or a centerline averageheight of no more than 1 nm as measured by the laser interferencemethod, and also in a method of manufacturing a ceramic mirrorcharacterized by the steps of compacting a titanium oxide powder havingan average particle diameter of no more than 1 μm and a purity of atleast 99% and sintering the compact in an ordinary, inert, vacuum, orreducing atmosphere at a temperature between 1000° and 1300° C.

DETAILED DESCRIPTION

The invention will now be described in detail.

In accordance with the invention a fine titanium oxide powder of highpurity is used as a material and is sintered at a lower temperature thanthe usual sintering temperatures to obtain a sintered body withexcellent surface smoothness, inhibiting the particle growth andpreventing the pore size increase while achieving an adequate sintereddensity.

The reason why a ceramic mirror having outstanding characteristics canbe obtained by the aforesaid method is theoretically to be clarifiedyet. Possibly the low temperature sintering of a fine titanium oxidepowder keeps the pores within the crystal grains (a phenomenon known as"ghosting"), suppressing the accumulation and growth to a grain boundaryphase.

The expression "a lower temperature than the usual sinteringtemperatures" as used herein means a temperature range from 1000° to1300° C. If the temperature is below 1000° C. the sintering itself doesnot proceed. A temperature above 1300° C., on the other hand, causesrapid grain growth and therefore a decrease in mechanical strength andpore migration and growth to a grain boundary phase. Consequentreduction of surface smoothness and irregular reflection of lightrenders it impossible to obtain a mirror of satisfactory opticalcharacteristics.

If the material powder is not at least 99% pure titanium oxide or has alarger average particle diameter than 1 μm, it is no longer easilysinterable at low temperatures. Sintering in the temperature range of1000° to 1300° C. will not give a product with an adequate sintereddensity.

For this reason it is desirable that the titanium oxide powder have apurity of at least 99%, preferably at least 99.8%, and an averageparticle diameter of no more than 1 μm, preferably no more than 0.5 μm.

The titanium oxide powder is compacted by CIP (cold isostatic pressing),mechanical pressing, doctor blade method, extrusion, or other techniquedepending on the configuration of the product. Whatever compactingtechnique is chosen, an organic binder or the like suited for theparticular method is used as a compacting assistant. The compact thusobtained is sintered in air, vacuum, or an atmosphere of N₂, Ar, H₂ orthe like, at sintering temperatures in the optimum range of 1000° to1300° C. Where greater surface smoothness is required, the compact ishot press sintered under a pressure of at least 50 kg/cm². Such asintered body is further treated by hot isostatic pressing (HIP) under apressure of not less than 500 kg/cm².

The TiO₂ sintered body so obtained is machined, e.g., by surfacegrinding or lapping, to a mirror for use in a laser or X-ray apparatus,camera, or other precision optical instrument. When necessary, themirror is coated with a metal of high reflectivity so as to be a mirrorfor a precision optical application.

Such a ceramic mirror has a surface flatness of no more than 1 μm and/ora centerline average height of no more than 1 nm. Moreover, the amountof its irregular reflection of incident light measured with acolorimeter is 5% or less. If the characteristic values exceed thenumerical values specified above, too much irregular reflection willresult, making the optical characteristics inadequate for a mirror.

BEST MODE FOR CARRYING OUT THE INVENTION Examples 1 to 4 of the presentinvention and Comparative Examples 5 to 8 are given below. Example 1

To a titanium oxide powder having an average particle diameter of 0.25μm and a purity of 99.8% ("CR-EL" (trade name), a product of IshiharaSangyo Kaisha) was added 2% polyvinyl alcohol (PVA) as a compactingassistant. After mixing in a mortar and drying, the mixture was sievedfor particle size regulation.

The mixture was compacted by CIP at a pressure not lower than 1000kg/cm² and sintered at ordinary pressure in air at 1200° C. The sinteredbody so obtained was lapped, and its surface flatness and amount ofirregular reflection were measured. The results are given in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                      Amount                                               TiO.sub.2 powder                                                                              Sur-                                                                             Center-                                                                             of ir-                                                  Aver.        face                                                                             line  regular                                     Sam-     Pu-                                                                              part.        flat-                                                                            aver. reflec-                                     ple      rity                                                                             dia.                                                                              Sintering con-                                                                         ness                                                                             height                                                                              tion                                        No.      (%)                                                                              (μm)                                                                           dition   (μm)                                                                          (nm)  (%)                                         __________________________________________________________________________    Example                                                                             (1)                                                                              99.8                                                                             0.25                                                                              Sintered at                                                                            0.9                                                                              0.8   4                                                           ordinary                                                                      pressure in air                                                               at 1200° C.                                                  (2)                                                                              "  "   Sintered at                                                                            0.8                                                                              0.7   3                                                           ordinary                                                                      pressure in Ar                                                                at 1100° C.                                                  (3)                                                                              "  "   Hot press                                                                              0.4                                                                              0.5   2                                                           sintered in N.sub.2                                                           at 1000° C.                                                  (4)                                                                              "  "   After sintering                                                                        0.2                                                                              0.3   1                                                           in air at                                                                     1200° C., HIP'd                                                        in Ar at 1000° C.                                                      and 1800 kg/cm.sup.2                                          Comp. Ex                                                                            (5)                                                                              96.0                                                                             0.25                                                                              Sintered at                                                                            1.8                                                                              3.2   10                                                          ordinary                                                                      pressure in air                                                               at 1200° C.                                                  (6)                                                                              "  "   Sample 5 1.4                                                                              2.7   8                                                           sintered in air                                                               at 1000° C. and                                                        HIP'd at 1800                                                                 kg/cm.sup.2                                                         (7)                                                                              99.5                                                                             1.5 Sintered at                                                                            2.4                                                                              6.5   15                                                          ordinary                                                                      pressure in air                                                               at 1200° C.                                                  (8)                                                                              "  "   Sample 7 sin-                                                                          2.0                                                                              4.1   13                                                          tered at 1000° C.                                                      and HIP'd at                                                                  1800 kg/cm.sup.2                                              __________________________________________________________________________

Example 2

The titanium oxide powder of Example 1 was used. Following compacting,it was sintered in an Ar atmosphere at 1100° C. The properties of thesintered body after lapping are shown at (2) in Table 1.

Example 3

Using the titanium oxide powder of Example 1, hot press sintering wasperformed in a N₂ atmosphere at 1000° C. and at a pressure of 200kg/cm². The sintered body after lapping exhibited properties as shown inTable 1.

Example 4

The sintered body obtained in Example 1 was treated by HIP in an Aratmosphere at 1000° C. and at a pressure of 1800 kg/cm². Table 1 givesthe properties of the sintered body as lapped.

Comparative Example 5

Sample No. 5 was obtained by following the same procedure as describedin Example 1 with the exception that a titanium oxide powder having anaverage particle diameter of 0.25 μm and a purity of 96.0% was employedinstead. The properties of the sintered body after lapping are shown inTable 1.

Comparative Example 6

Sample No. 5 was compacted and sintered in air at 1000° C. and atordinary pressure and then treated by HIP under the conditions given inExample 4 to obtain Sample No. 6. The sintered body upon grinding showedproperties as in Table 1.

Comparative Example 7

Except for the use of a titanium oxide powder having an average particlediameter of 1.5 μm and a purity of 99.5%, the procedure of Example 1 wasrepeated, when Sample No. 7 was obtained. The sintered body propertiesafter grinding are given in Table 1.

Comparative Example 8

Sample No. 7 was sintered in air at 1000° C. and at ordinary pressureand treated by HIP under the conditions of Example 4 to obtain SampleNo. 8. Table 1 shows the sintered body properties after grinding.

Industrial Applicability

As described above, sintering a high-purity, minute titanium oxidepowder at a low temperature below the usual sintering temperature rangemakes ghosting of pores in the resulting sintered body possible. In thisway titanium oxide sintered parts have been obtained which exhibit highenough surface smoothness and optical characteristics for mirrors to beused in laser and X-ray apparatuses, cameras, and other precisionoptical instruments.

I claim:
 1. A method for manufacturing a ceramic mirrorcomprising:compacting a titanium oxide powder having an average particlediameter no greater than 0.5 μm and a minimum purity of 99.8%; sinteringsaid compacted titanium oxide powder to produce a ceramic body atordinary pressure and a temperature between 1000° C. and 1300° C. inair; and machining said ceramic body by surface grinding and lapping toproduce a ceramic mirror having a maximum mirror surface flatness of 1μm as measured by a laser interference method.
 2. A method formanufacturing a ceramic mirror comprising:compacting a titanium oxidepowder having an average particle diameter no greater than 0.5 μm and aminimum purity of 99.8%; sintering said compacted titanium oxide powderto produce a ceramic body at ordinary pressure and a temperature between1000° C. and 1300° C. in air; and machining said ceramic body by surfacegrinding and lapping to produce a ceramic mirror having a mirror surfacewith a centerline average height no greater than 1 μm as measured by alaser interference method.
 3. A method for manufacturing a ceramicmirror as claimed in claim 1 wherein said machining furthercomprises:producing a ceramic mirror having a mirror surface with acenterline average height no greater than 1 nm as measured by a laserinterference method.
 4. The method as claimed in claim 1 and furthercomprising:hot isostatic pressing said ceramic body after said sinteringat a temperature between 800° C. and 1500° C. under a minimum pressureof 500 kg/cm².
 5. The method as claimed in claim 1 wherein:saidcompacting comprises cold isostatic pressing at a minimum pressure of1000 kg/cm² ; and said sintering temperature is 1200° C.
 6. The methodas claimed in claim 1 wherein:said sintering temperature is 1100° C. 7.The method as claimed in claim 4 wherein:said hot isostatic pressing isin an argone atmosphere at a temperature of 1000° C. and a pressure of1800 kg/cm².